The present invention relates to intrinsic safety, and more particularly to an intrinsically safe galvanically isolated barrier device.
Intrinsic safety is a protection concept deployed in sensitive and potentially explosive atmospheres. Intrinsic safety relies on equipment designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas. Intrinsic safety can be achieved by limiting the amount of power available to the electrical equipment in a hazardous area to a level below that which will ignite the gases. In order to have a fire or explosion, fuel, oxygen and a source of ignition must be present. An intrinsically safe (IS) system that operates in an atmosphere where fuel and oxygen are present is designed such that the electrical energy or thermal energy of a particular instrument loop can never be great enough to cause ignition. There are various IS standards set forth by various certifying agencies for a system to be considered IS. Such standards include International Electrical Commission (IEC) IEC 60079-11, Factory Mutual (FM) 3610, Underwriters Laboratories (UL) UL913, etc.
In many cases, equipment in a hazardous area that is required to be IS, must connect to non-IS equipment located in a non-hazardous area. In these cases, the IS equipment and non-IS equipment can connect through an IS barrier. IS barriers are typically located at a border of a hazardous area and non-hazardous area and ensure that all electrical signals flowing between the non-IS equipment and the IS equipment are IS. Two types of IS barriers available for this purpose are non-galvanically isolated barriers and galvanically isolated barriers.
Non-galvanically isolated barriers are commonly implemented as a diode barrier, typically with an arrangement of fuses, rectifier diodes, resistors, and zener diodes. There are many commercially available IS certified devices on the market from suppliers such as Pepperl+Fuchs, Stahl, Phoenix Contact, MTL, etc. Non-galvanically isolated barriers limit or shunt the available energy from a non-IS system down to levels which are considered safe under IS standards. However, non-galvanically isolated barriers have certain limitations. One such limitation is that a non-galvanically isolated barrier must have a safety ground which provides an infallible connection to an earth ground and the power system ground. Since the barrier is grounded and non-isolated, due to IS requirements for safety, no other point in the IS system may be grounded. This may present challenges to designing a system including many pieces of equipment with various levels of power consumption. Electromagnetic compatibility (EMC) is also much more difficult when local grounding is not possible. The requirement that the non-galvanically isolated barrier be the only grounded point in the IS system also can limit how the IS system is powered. IS power sources are commercially available for use in IS systems, however, the flexibility of use of such devices is greatly reduced when applied in a non-galvanically isolated barrier system. This can lead to high costs and a lack of system flexibility.
Galvanically isolated barriers provide a method of communication between non-IS equipment and IS equipment without any electric current flowing from the non-IS side to the IS side. For example, optical couplers which transmit information as light waves are commonly used in the construction of galvanically isolated barriers in order to provide isolated signal coupling. Such optical couplers may include a light source, such as an LED, and a photosensitive device to receive light emitted from the light source, such as a photoresistor. Unlike non-galvanically isolated barriers, galvanically isolated barrier devices do not need a safety ground. Since galvanically isolated barriers have no low impedance connection between the IS and non-IS equipment, such barriers allow greater flexibility in designing the IS system. Galvanically isolated barriers are commercially available, but typically at greater cost than non-galvanically isolated barriers. However, in many cases, the increased expense can be offset by reduced cost impact to the rest of the system. In addition to cost, typical currently available galvanically isolated barriers have other drawbacks as well. One such drawback is slow data transmission rates. This is due at least in part to the fact that typical galvanically isolated barriers rely on optical coupling, which is inherently slow when compared to state of the art data rates. For example, in current commercially available galvanically isolated barriers, data transmission is limited to approximately 20 kbits/second. Furthermore, separate power sources are often required on both sides (IS and non-IS) of the barrier, so an IS power supply may be used to power equipment on the IS side of the barrier.
In the process control industry, process analysis equipment with associated process sample handling systems are used to monitor and control chemical processes. The New Sample/Sensor Initiative (NeSSI) is an initiative which promotes the use of modular sample system component technology to implement sample systems that are associated with analytical process equipment. NeSSI Generation 2 involves intelligent control of modular sample systems. In order to achieve intelligent control of modular sample systems, two-way communication must be established between the modular devices that make up the sample system and the control device. Accordingly, a digital communication bus can be used to connect the modular devices so that the modular devices can communicate using a protocol. Examples of protocols used to implement NeSSI sample systems include Controller Area Network (CAN), I2C, RS-485 based protocols such as Profibus or Fieldbus, RS-232 protocols, etc.
In process sample systems in which IS modular devices are located in a hazardous area and non-IS devices, such as a sample system controller, are located in a safe area, it is desirable to connect the IS modular devices in the hazardous area with the non-IS devices in the safe area through a serial communication bus. This serial communication bus must connect through an IS barrier located at a border of the safe area and the hazardous area. Due to the disadvantages of non-galvanically isolated barriers and the advantages of galvanically isolated barriers described above, it may be desirable to galvanically isolate the IS devices from the non-IS devices. However, as described above many conventional galvanically isolated IS barriers operate at insufficient data rates to efficiently handle the data demands of modern sample systems, and may increase costs required to implement the sample system. Accordingly, a need exists for a high speed (high date rate), low cost, galvanically isolated barrier for use with a digital communication bus, such as a NeSSI Generation 2 sample system bus. The term NeSSI Generation 2 can be paraphrased as the common mechanical substrates defined by NeSSI with the additional layer of a digital communication bus and power that connects these mechanical devices, the communication layer explicitly meaning Generation 2.
Recently, in order to address the above described need, developments have been made in high speed digital isolator technology. Monolithic integrated circuit isolation devices have been developed that transfer data at rates of greater than 10 Megabits/second through miniature monolithic transformers or capacitors using modulation techniques. These devices are monolithic integrated circuits, and are thus relatively inexpensive. Examples of such devices include the Texas Instruments ISO721 and the Analog Devices ADuM1200, ADuM1300, and ADuM1400 series devices.